PETCAT: A Key System to Upcycling PET Bottles

Introduction

Every year, millions of tons of plastic waste end up in landfills and oceans, harming wildlife and polluting our environment. One of the most common plastics is polyethylene terephthalate (PET), the material used to make water bottles, food containers, and many packaging products. Although PET can be recycled, traditional recycling methods often require high temperatures, consume large amounts of energy, and reduce the quality of the plastic after repeated recycling.

Imagine if tiny living cells could do the recycling for us instead. Even more exciting, what if they could transform old plastic bottles into useful chemicals that are used to make medicines, fragrances, and other valuable products? This is exactly what researchers have achieved in a groundbreaking study. By combining enzyme engineering, microbiology, and synthetic biology, scientists have developed an innovative system that not only breaks down PET plastic but also converts it into a high-value chemical. This remarkable discovery represents an important step toward greener, more sustainable recycling technologies and demonstrates the incredible potential of biotechnology to solve environmental challenges.

The Challenge of Plastic Pollution

Plastic has become an essential part of modern life because it is lightweight, durable, and inexpensive. However, these same properties also make it difficult to dispose of. PET plastics can remain in the environment for hundreds of years before naturally breaking down. Conventional recycling methods have several drawbacks. Mechanical recycling often lowers the quality of the plastic, making it unsuitable for repeated use. Chemical recycling can recover the original building blocks of PET but usually requires high temperatures, expensive equipment, and significant energy consumption. These limitations encourage scientists to search for cleaner and more sustainable alternatives.

One promising approach is enzymatic recycling, where natural proteins called enzymes break down plastic into its original chemical components. Enzymes work under much milder conditions than industrial processes, making them an environmentally friendly option. The researchers focused on an enzyme called MG8, which was originally discovered in microorganisms living in the human saliva microbiome. This enzyme naturally has the ability to break down PET plastic at moderate temperatures, making it suitable for use inside living cells. However, the natural version of MG8 was not efficient enough for practical applications. To improve its performance, the scientists first determined its three-dimensional crystal structure. By examining the enzyme at the atomic level and using computer simulations, they identified flexible regions that could potentially be modified to increase its activity.

The research team then created more than 1,000 different versions of the enzyme by introducing small genetic changes into these flexible regions. Using a high-throughput mass spectrometry screening system, they rapidly tested which enzyme variants were best at degrading PET. After extensive testing, two engineered versions stood out. These modified enzymes showed dramatically improved ability to break down PET at 37°C, which is close to normal human body temperature and much lower than the temperatures required by many other plastic-degrading enzymes.

Teaching Bacteria to Recycle Plastic

Improving the enzyme was only the first step. The researchers wanted to create a complete biological recycling system using living bacteria. They engineered Escherichia coli (E. coli), a commonly used laboratory bacterium, to produce and release the improved MG8 enzyme into its surroundings. Instead of purifying the enzyme separately, the bacteria continuously produced fresh enzymes while remaining alive.

This approach offered several advantages. Fresh enzymes were constantly supplied, replacing older enzymes that gradually lost activity. As a result, the living bacterial system broke down PET more efficiently than purified enzymes alone. During experiments, the bacteria successfully degraded low-crystallinity PET materials, similar to those found in many disposable plastic products. The PET was converted into smaller molecules, including terephthalic acid (TPA), one of the main building blocks of PET plastic.

From Plastic Waste to Valuable Chemicals

Perhaps the most exciting part of the research was what happened next. Rather than stopping after breaking down the plastic, the scientists engineered a second group of bacteria with seven additional genes that could convert terephthalic acid into catechol. Catechol is an important industrial chemical used to manufacture pharmaceuticals, fragrances, pesticides, and many other products. Instead of treating plastic waste as rubbish, the researchers transformed it into a valuable raw material for entirely different industries.

The scientists combined the two groups of bacteria into a system they called PETCAT. The first bacterial strain, known as the "PET cells," degraded the plastic and released terephthalic acid. The second strain, called the "CAT cells," immediately converted that terephthalic acid into catechol. This two-step biological process represents an example of upcycling, where waste materials are transformed into products with greater value than the original material.

Why This Research Is a Green Solution

This research offers several environmental advantages compared with conventional recycling methods. First, the entire process operates at relatively low temperatures, reducing energy consumption and lowering greenhouse gas emissions. Instead of using large industrial furnaces, the engineered bacteria perform the recycling under conditions similar to those found in nature. Second, the process avoids many harsh chemicals commonly used in chemical recycling, making it more environmentally friendly. Third, the system converts plastic waste into useful chemicals instead of simply producing recycled plastic with reduced quality. This creates additional economic value while reducing dependence on petroleum-based raw materials. Finally, the research demonstrates how living cells can perform multiple tasks simultaneously, continuously producing enzymes while converting waste into valuable products. This could eventually make recycling more efficient and sustainable.

Challenges That Still Remain

Although the results are highly promising, the technology is not yet ready for large-scale industrial use. One limitation is that the engineered enzyme works best on PET with relatively low crystallinity. Highly crystalline plastics, such as many commercial bottles, remain more difficult to degrade. Scientists will need to continue improving the enzymes or develop better pretreatment methods to make these plastics easier to recycle. Another challenge is increasing the overall speed and efficiency of the process. Industrial recycling facilities must process enormous quantities of plastic every day, so future research will focus on making the biological system faster and more robust. The researchers also suggest that additional engineered bacteria could eventually recycle other plastic components, creating complete microbial communities capable of processing different types of plastic waste.

Conclusion

This innovative research demonstrates how biotechnology can provide creative solutions to one of the world's biggest environmental problems. By engineering both enzymes and bacteria, scientists created a living system capable of breaking down PET plastic and transforming it into a valuable industrial chemical. The study combines structural biology, enzyme engineering, synthetic biology, and metabolic engineering into a single powerful recycling platform. Instead of viewing plastic waste as an environmental burden, the researchers have shown that it can become a valuable resource for producing useful chemicals. Although further improvements are needed before the technology reaches commercial scale, this work represents an important milestone in sustainable recycling. It highlights how science can imitate and improve upon nature to solve global challenges.

References

Amornloetwattana, R., Eiamthong, B., Meesawat, P., Bunkum, P., Royer, B., Zeballos, N., Valenzuela‐Ortega, M., Robinson, R.C., Wallace, S. and Uttamapinant, C. (2025). Cellular Upcycling of Polyethylene Terephthalate (PET) With an Engineered Human Saliva Metagenomic PET Hydrolase. ChemSusChem, [online] 19(1). doi:10.1002/cssc.202502560.

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This article was prepared by Ulya Ammar (University of Edinburgh).

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